1. Field
The present invention relates generally to data communication, and more specifically to techniques for allocating resources in multiple-input multiple-output communication systems that utilize orthogonal frequency division multiplexing (i.e., MIMO-OFDM systems).
2. Background
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for transmission of multiple independent data streams. In one MIMO system implementation, at any given moment, all of the data streams are used for a communication between a multiple-antenna base station and a single multiple-antenna terminal. However, in a multiple access communication system, the base station may also concurrently communicate with a number of terminals. In this case, each of the terminals employs a sufficient number of antennas such that it can transmit and/or receive one or more data streams.
The RF channel between the multiple-antenna array at the base station and the multiple-antenna array at a given terminal is referred to as a MIMO channel. The MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A wideband MIMO system typically experiences frequency selective fading, which is characterized by different amounts of attenuation across the system bandwidth. This frequency selective fading causes inter-symbol interference (ISI), which is a phenomenon whereby each symbol in a received signal acts as distortion to subsequent symbols in the received signal. This distortion degrades performance by impacting the ability to correctly detect the received symbols.
Orthogonal frequency division multiplexing (OFDM) may be used to combat ISI and/or for some other purposes. An OFDM system effectively partitions the overall system bandwidth into a number of (NF) frequency subchannels, which may be referred to as subbands or frequency bins. Each frequency subchannel is associated with a respective subcarrier on which data may be modulated. The frequency subchannels of the OFDM system may also experience frequency selective fading, depending on the characteristics (e.g., the multipath profile) of the propagation path between the transmit and receive antennas. With OFDM, the ISI due to frequency selective fading may be combated by repeating a portion of each OFDM symbol (i.e., appending a cyclic prefix to each OFDM symbol), as is known in the art.
For a MIMO system that employs OFDM (i.e., a MIMO-OFDM system), NF frequency subchannels are available for each of the NS spatial subchannels of a MIMO channel. Each frequency subchannel of each spatial subchannel may be referred to as a transmission channel. Up to NF·NS transmission channels may be available for use at any given moment for communication between the multiple-antenna base station and the multiple-antenna terminal.
The MIMO channel between the base station and each terminal typically experiences different link characteristics and may thus be associated with different transmission capabilities. Moreover, each spatial subchannel may further experience frequency selective fading, in which case the frequency subchannels may also be associated with different transmission capabilities. Thus, the transmission channels available to each terminal may have different effective capacities. Efficient use of the available resources and higher throughput may be achieved if the NF·NS available transmission channels are effectively allocated such that these channels are utilized by a “proper” set of one or more terminals in the MIMO-OFDM system.
There is therefore a need in the art for techniques to allocate resources in a MIMO-OFDM system to provide high system performance.